Virus latency: Heterogeneity of host-virus interaction in shaping the virosphere

Viruses and viroids that are not causing symptoms on their initial host plant are described by the terms “latent”, “cryptic” or “symptomless” and represent 6% and 2% of the classified plant viruses and viroids, respectively. Additionally, endogenous plant pararetroviruses that reside in their host genome are often dormant and therefore asymptomatic. Improved sequencing and diagnostic technology demonstrated that viral sequences are ubiquitously present in the biosphere even if the signs or symptoms of infection are not obvious. Accordingly, the number of latent plant viruses listed today does not reflect the true amount of latent viruses existing in plants. Biodiversity within the virosphere comprises virus-host, virus-vector, virus-virus and virus-viroid interactions. Invasion of new host plants, climatic changes and changes in plant production and distribution can have a major impact on symptomatic outbreaks of otherwise latent viruses which seem incapacitated but remain potentially undefeated. Here we describe for selected ornamentals how these changes can induce latent viruses and report on possible underlying biochemical mechanisms. Comparison of virus latency in plant and animal hosts indicates that epigenetic modifications are an important factor for regulation in both systems.